The present invention relates, in general, to a transformer for coupling RF energy from an antenna to a receiver and, more specifically, to a composite transformer having input and output windings and a magnetizing loop winding for controlling coupling between the input and output windings.
Broadcast radio receivers, such as the superheterodyne receiver, typically include an antenna connected to a radio frequency (RF) amplifier followed by intermediate frequency (IF) stages, a detector, an audio amplifier, and an audio output transducer. Nearly all radio receivers employ automatic gain control (AGC) in their RF amplifier and IF amplifier stages. The use of AGC ensures that an amplifier output remains at nearly a constant level in spite of variations in the strength of incoming signals.
The control voltage for AGC action is normally derived from the detector stage of the receiver. The AGC voltage is a DC voltage which is determined by the strength of the received carrier signal and is used to inversely affect the gain factor of the associated RF or IF amplifier. As a result of automatic gain control, the average input signal power to the audio amplifier stage has a constant level although receiver motion and external conditions may cause variations in received RF signal strength.
Although the RF amplifier of a typical receiver is responsive to input signals over a range of magnitudes, it is fairly easy to overload the RF amplifier. For example, a typical RF amplifier can be overloaded by an antenna signal strength of as little as 50 millivolts. The overloaded amplifier becomes nonlinear, thus introducing harmonic distortion.